Apparatus for and method of modulating waves



A ril 21, 1942. I J, EVAN-5 2,280,693

APPARATUS FOR AND METHOD O MODULATING WAVES Filed June l, 1940 2 Sheets-Sheet l M: gs t A R 5; 3 mwflg wt: Q 2' 1$ U R: q

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Zhwentor John Evans Bg (Ittotneg April 21, 1942. EVANS I 2,280,693

APPARATUS FOR AND METHOD OF NODULATING WAVES Filed June 1, 1940 2 Sheets-Sheet 2 MODULH'I'IO/V I Vol 'mrGL' I 25 g 50 1 26. 3. I 1 I Q I Y: 100 I 'I'IML' h 31 A'i j 'm ll 26' i *H/37 I A A F 3nnentor John Evans attorney Patented Apr. 21, 1942 APPARATUS FOR AND METHOD OF MODULATING WAVES John Evans, Palmyra, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 1, 1940, Serial No. 338,342 9 Claims. (or. 179-1715) This invention relates to an apparatus for and a method of modulating. More especially the invention includes the apparatus for and .the method' of phase or frequency modulating in which the unmodulated transmission is at a stabilized frequency.

Strictly speaking, in frequency modulation transmission no carrier wave is used as con trasted to amplitude modulation in which the carrier frequency is varied in amplitude as a function of the applied modulation. In the case of amplitude modulation systems the carrier frequency is maintained constant within a few parts "in a million. In a case of frequency modulation the wave to be transmitted is changed in frequency as a function of the applied modulation. In systems of this type, it is highly desirable to maintain the unmodulated wave at a constant frequency.

It is, therefore, an object of this invention to provide means for phase or frequency modulating a wave to be transmitted. Another object is to provide means for phase or frequency modulating a wave to be transmitted and at the same time maintaining substantially constant the unmodulated wave. Another object is to provide a method for. phase or frequency modulation in which the phase or frequency of the transmitted wave is varied as a function of.the applied modulation, whilethe unmodulated wave is maintained at substantially constant frequency.

The invention will be described by referring to the accompanying drawings in which Figure 1 is a schematic circuit diagram of one embodiment of the invention, and Figures 2 and -3 are graphs illustrating the operationof the circuits of the invention.

A constant frequency generator I such as a piezo-electric oscillator is connected to a multivibrator 3. The multivibrator includes a pair of thermionic tubes 5, I which are connected in a manner well known to those skilled in the art. The output of the multivibrator is coupled to the input circuit of a class B amplifier 8. The output circuit of the class B amplifier includes a resonant circuit II. The resonant circuit is coupled to one or more frequency multiplier stages l3 which may also serve as limiters. The output of the frequency multiplier is applied to a load circuit, such as an antenna. A modulator I! is provided with an input circuit connected to a source of modulation indicated by reference numeral l'l with an output circuit connected to the input circuit of the class B amplifier 9.

The operation of the circuit is as follows: The

constant frequency generator I produces currents of sine wave form I9. These currents are applied to the input of the multi-vibrator which generates currents of saw-tooth wave form 29. The frequency of the saw-tooth wave corresponds to that of the constant frequency generator I. In the absence of modulation, the class B amplifier 3 is biased so negatively thatonly the peaks of the saw-tooth wave 29 operate the amplifier. These peaks cause pulses of current to be applied to the resonant circuit ll. Since the frequency of the pulse peaks is determined by the frequency of the generator I, it follows that the output of the resonant circuit II is of constant frequency and of substantially sinusoidal wave form 23. The sine wave currents 23 are applied to the frequency multiplier l3 which may also operate as a limiter. The'frequency multiplier and limiter produces in its output circuit waves of multiplied frequency in which the peaks are flattened by the limiting action as shown by the curve 24. Thus, in the absence of modulation or in the unmodulated state, a wave of constant frequency is transmitted.

When modulation is applied the bias on the amplifier 9 ismade less negative. Referring to Fig. 2, it will be seen that in the absence of modulation only the peak of the saw-tooth wave 21 is applied with the result that the wave E0 is established in the resonant circuit of the amplifier. When the modulation voltage V1 is applied not only isthe larger portion of the saw-tooth wave 21 utilized but the resultant wave EI begins at a different time. As the modulation voltage is increased to V2 a still larger peak is applied and the generated wave in the plate circuit of the amplifier corresponds to E2. Upon application of a modulating voltage V3 the wave E3 is formed. It will beobserved that the several waves E0 to E3 have a phase which varies as a function of the applied modulation.

It will be observed that while the resulting waves vary in amplitude, as well asin phase, the variation in amplitude will be eliminated by the limiting action of the frequency multipliers. This the class B amplifier 9 as a function of the modulation voltage. The modulation voltages are chosen as 25, 50, '75 and 100 volts, by way of example. The plate currents are represented by the broken line curves ll at the base of the fi ure. If the modulation voltage is zero, only the first square wave 33 will appear. As the modulastant frequency generator. The currents fromthe constant frequency generator are used to control a saw-tooth wave which is applied to a class B amplifier along with the modulating signal current. The phase of the applied currents varies because of the saw-tooth wave form. The currents applied to the amplifier establish sinusoidal currents in the tuned circuits of the amplifier. These currents which vary in phase or frequency'may be frequency multiplied and limited before they are applied to the load circuit. While the multivibrator has been shown as the generator of the saw-tooth waves, it should be understood'that any saw-tooth wave generator may be employed. Furthermore, in place of the crystal oscillator, any generator or source of currents of stable frequency may be used. Furthermore, the system will operate with any of the well known types of frequency multipliers and limiters. If phase modulation is employed, no compensating network is required in the modulator. If frequency modulation is used, the modulator should include the well known delay network.

I claim as my invention:

1. The method of modulating which includes generating frequency stabilized currents of substantially sinusoidal wave form, converting said currents of sinusoidal wave form into currents of saw-tooth wave form, combining said currents of saw-tooth wave form with currents corresponding to the modulation signals to obtain pulses of current or variable amplitude and variable duration, applying said current of variable amplitude and variable duration to a resonant circuit, and obtaining from said resonant circuit currents of substantially constant amplitude'and of a phasev varying as a function of said modulation signals.

2. Themethod of modulating which includes generating frequency stabilized currents of substantially sinusoidal wave form, converting said currents into currents of saw-tooth wave form, establishing currents corresponding to the desired modulation, combining said currents of saw-tooth wave form with said currents corre-- sponding to the modulation to obtain pulses of current of variable amplitude and variable duration, applying said current of variable amplitude and variable duration to a resonant circuit, and obtaining from said'resonant circuit currents of substantially constant amplitude and of a phase varying as a function of said modulation signals.

3. The method of modulating which includes generating frequency stabilized currents of substantially sinusoidal wave form, converting said currents into currents of saw-tooth wave form,

establishing currents corresponding to the desired modulation, combining said currents of sawtooth wave form with said'currents corresponding to the modulation to obtain pulses of current of variable amplitude and variableduration, applying said current of variable amplitude and variable duration to a resonant circuit, and obtaining from said resonant circuit currents of substantially constant amplitude and of a frequency varying as a function of said modulation signals.

4. The method of modulatingwhich includes generating frequency stabilized currents of substantially sinusoidal wave form, converting said currents into currents of saw-tooth wave form, establishing currents corresponding to the desired modulation, combining said currents of saw-tooth wave form with said currents corre sponding to the modulation'to obtain pulses of current 'ofvariable amplitude and variable duration, applying said pulses of current to a resonant circuit to convert said pulses of current into currents of substantially sinusoidal wave form having a phase varying as a function. of said modulation, multiplying the frequency of the lastnientioned currents and limiting the amplitude of the frequency multiplied currents.

5. The method of modulating which includes generating frequency stabilized currents of substantiallysinusoidal wave form, converting said currents into currents ofsaw-tooth wave form, deriving modulation currents corresponding to the desired modulation, combining said currents of saw-tooth wave form with said modulation currents to obtain currents of variable amplitude and-variable duration, applying said currents of variable amplitude and variable duration to a resonant circuit to obtain currents of sinusoidal wave form, and deriving from said last-mentioned currents, currents of flattened wave form having a frequency variation corresponding to the modulation signals and an unmodulated frequency corresponding to said stabilized frequency.

6. The method of frequency modulating described by claim 5 including the final step of frequency multiplying the last-mentioned currents.

7. A device for frequency or, phase modulating including in combination a source of currents of stabilized frequency, a saw-tooth wave generator connected to said source, a source of modulating currents, means connected to said generator and said modulating current source and including a resonant circuit for combining said saw-tooth and modulating currents of variable amplitude and variable duration, and means including said combining means for converting the combined currents into currents having a flattened wave form and a frequency variation corresponding to said modulating currents.

8. A device of the character of claim '7 including a frequency multiplier stage effectively connected to the last-mentioned means.

9. A device of the character of claim 7 including a frequency multiplier connected to said last mentioned means and includingmeans for limiting the amplitude of the currents derived from said frequency multiplier.

JOHN EVANS. 

